A discharge laser typically consists of two mirrors, concave or flat, defining an optical resonator cavity coupled together with a discharge defining an optical path between the reflectors.
The discharge is typically a channel ground into a ceramic block (e.g. aluminum oxide, Al2O3) with a lower electrode of aluminum or copper added to complete a cross-section of the discharge. Alternatively, the discharge can be ultrasonically drilled down through a piece of ceramic such as aluminum oxide (Al2O3) to create a continuous closed bore length with upper and lower electrodes parallel to the bore length. Typically, the positive arm of the oscillating electromagnetic field (e.g. Radio Frequency—RF) supply will be coupled into the upper electrode of the discharge, and the ground plane of the RF supply will be coupled to the lower electrode. Resonance is added between and along the length of the upper electrode to distribute the RF standing wave evenly along the length of the electrodes. Finally, the mirrors and discharge structure are aligned and housed in a vacuum vessel (laser housing) that holds the gas to be excited.
Discharge lasers suffer from the disadvantage that, for the lengths needed, the discharges are difficult to fabricate with sufficient accuracy at a reasonable cost to obtain acceptable laser performance. It is very difficult to cost-effectively fabricate a typical discharge structure that is roughly 30 to 40 cm long with a 1.5 to 3.0 mm bore. Bore cross-section inaccuracy leads to unacceptable laser transverse mode characteristics and reduced power output. Due to the size, current ceramic discharges are constructed by casting or extruding. Casting tolerances are high, requiring expensive machining (grinding) after the piece is formed to acquire the desired accuracy.
Additionally, a discharge laser is a balance between the loss in its inherent internal RF circuit and heat removal efficiency. Ideally, to minimize the RF losses the capacitance between the top and bottom electrodes (RF+ and RF− or ground) needs to be high, which translates into using as little ceramic as possible in the discharge sidewalls. With Al2O3, thermal efficiency requirements dictate the use of a large ceramic area, which creates a lossy RF circuit. Ideally materials with good thermal properties such as BeO and AlN are desirable as the ceramic, but are prohibitively expensive with related art discharge designs.
Additionally the resonator cavities of discharge lasers suffer energy losses from misalignment of the containment mirrors, impingement of laser with the discharge walls, and reflectivity properties of the containment. For example the use of planar mirrors at either end of the resonator cavity, unless perfectly aligned, enable only a limited number of reflections. Thermal heating in high powered lasers can distort the reflectivity properties resulting in laser degradation.
Since the bore cross-sections, in the related art, are the result of grinding or ultrasonic drilling, most bores are either rectangular or circular. This results in bores that are optimized for the manufacturing process rather than the optical properties of the device. For example, the use of curved containment mirrors results in variable beam radius throughout the resonator cavity, thus the discharge channels of related art fail to allow the optimization of the discharge with respect to variable beam radius in the resonator channel.
In related art, the electrode positioning, and subsequent resonance electric field generation, is partly a function of the electrode spacing, and is often determined by the size of the discharge structure (i.e. the distance between electrodes). Various spacing between electrodes results in varying power levels and current. Current related art fails to fully optimize the electrode spacing and instead conventional methods focus on ease of manufacturing instead of optical optimization since a portion of the spacing is filled with the ceramic discharge structure.
Additional problems exist in conventional gaseous lasers, for example, laser startup. Traditional CO2 lasers are started at 70-80 torr and have difficulty starting without some manipulation of the system. The higher the startup pressure the higher the efficient use of the lasing medium. but the total power emitted decreases.
A related art system is described in Laakmann (U.S. Pat. No. 4,169,251). Laakmann is directed to a conventional Closely Coupled RF Excited (CCRFE) laser that suffers from many of the same problems as other conventional systems.
There are many uses for multiply combined laser systems. For example missile defense systems, atmospheric sampling, communication correction, etc. . . . For example, shoulder launched infrared missiles can have an infrared sensors in them to detect heat energy in the 3 to 5 and/or 8 to 12 micrometer wavelength band. Other energies used include the ultraviolet (UV) energy spectrum, which is below the visible spectrum. The near, mid and far infrared spectrums are above the visible spectrum in that respective order. Prospective targets, such as Jet engines, and the like, all emit much higher levels of infrared energy than visible energy (especially at night). So missiles that sense and track the infrared energy were developed in the early to mid 60s and introduced in the late 60s, such as the Redeye missile. The earliest missiles could use a four-quadrant detector, essentially a four-pixel detector in a 2×2 matrix. When the missile sees a shift in the energy from one pixel to the next (one quadrant to the next) the missile will turn slightly to recenter the energy (the energy would be equal on all four pixels). The missile has a viewing window of about 3 degrees so it does not have to steer much. Thus, the missile has to be launched straight at the target. The detectors also have a spinning wheel in front of the detector with a small hole in the wheel so the infrared energy only hits the pixels for the duration of the hole diameter per every rotation of the spinning wheel. The wheel allows the input from the detector to go from zero to full input power for each rotation (more signal strength), and also gives the sensor a time base (the wheel's spin is driven by the missile speed). The rate at which the infrared signal was acquired and tracked allows the missile to differentiate a decoy like a flare. A flare can be made to have the same infrared output of an engine, but flares do not move like engines (flares quickly slow down once ejected to gain separation from the jet) so the energy growth rate as the missile approaches the engine and flare would be different. The difference in the rate of infrared energy growth allows the missile to determine which infrared source is the engine versus the flare.
In order to make the missile more intelligent a second sensor can be added that looks for UV energy (and the name was changed to the Stinger missile in the mid 70s). Missiles now look at the infrared signal and confirms that there is an associated UV signal (referred to as ‘two color’ detectors by loosely borrowing the two wavelength phrase from the visible spectrum). Engines emit both types of energy. The UV sensor is only a signal input and all of the tracking is still done with the infrared energy detector.
Updated missiles can use focal plane arrays (array of 128×128 pixels or even up to 1024×1024) so that there is an actual picture of the target transmitted to the missile CPU. Algorithms are added to ignore the sun, flares, and even a friendly target. This technology was added in the mid 80s, and is being expanded on today.
The British and the Swedes build their missiles with an optical output back to the person on the ground. The ground personnel then look at the infrared signal and manually steer the missile. The problem with the optical maneuvered missiles is that they are extremely hard to launch and steer so the missile hit rate drops significantly.
The task becomes how to fool or defeat the enemy infrared missile. Initially CO2 gas lasers were built that emit infrared energy at the correct wavelength to be added to the spinning wheel detector's input to cause a signal that the missile misreads, which causes the missile to veer off or explode prematurely because of the errant infrared signal. When the two color missiles became prevalent, Northrup Grumman (NG) and British Aerospace (BAE), formerly Sanders Corporation in NH, switched from CO2 or gas laser jammers to solid state crystal laser jammers. NG has switched over to YAG crystal technology and has all but closed their CO2 business and BAE Sanders never started a CO2 gas laser group because they assumed that the YAG solid state would replace it.
The solid state crystal laser jammers are based on a diode pumped YAG crystal and are much more frequency agile. Thus the laser can output both the infrared jamming signal and the UV signal. The YAG lasers can only jam the missiles detectors so they are useless against both the British/Swede type of design and are also useless against the new focal plane array detector technology.
Introduction for Laser Fusion Systems
The National Ignition Facility (NIF) is a facility constructed at the Lawrence Livermore National Laboratory (LLNL) for the study of laser fusion. The NIF facility has 192 laser beams, providing 1.8 megajoules, 500 terawatts for 3 nanoseconds, at 351 nm laser. The 192 lasers compress small fusion targets to conditions in which they ignite and burn. Krypton-Flouride lasers are being considered for use in laser fusion.
The Krypton-Flouride laser uses a high-voltage pulsed-power source, which generates a uniform electron beam from the cathode. The electron beam propagates through the foil support and deposits its energy in the laser cell, filled with krypton, fluorine and argon gases. A complex set of ionizations and chemical reactions produce the excited molecular state of KrF*. The input laser beam then stimulates the decay of this molecule to its ground state of separate atoms, with an enhancement of the laser intensity with the formulation:Electron Energy+Kr+F2→KrF*+F→Kr+F2+light  (1)
Lasers are used to heat fuel pellets because the time duration can be kept low, thus the power can be increased. The formulation for power is:power=energy/(time duration)  (2)
Thus, if the energy is constant but the time duration can be decreased the power increases. For conventional fusion laser systems the energy in each shot is not very large (1.5 megajoules) but the duration is very short (nsec).
Introduction for Laser Communication Systems
Free-space laser communications has been in existence for decades. After the first demonstration of the laser in 1960, papers and patents addressing the feasibility of laser communication appeared two years later. Early attempts include reflecting signals from metallic coated, inflated satellites and hand-held communicators from the first manned space efforts.
The NASA deep space network is a region where the use of laser communications can be used to alleviate the current capacity limitations. To prevent a crisis that would leave valuable data stuck in outer space, the radio transmissions that download information from orbiting satellites and more distant spacecraft may be gradually replaced with laser-light-digital delivery. NASA is looking at near infrared lasers since the observation telescopes are already in existence. Further examples exist such as secured communications amongst military satellites.
Introduction to Scaling and Discharge
Problems exist in scaling up low power rf-excited gas discharge lasers to high power ones. The rf power distribution tends to become uneven over the discharge area and instead concentrates in one spot, thereby ruining what should otherwise have been a uniformly excited discharge suitable for efficient laser power extraction. It is known that this problem can be solved by making a high power laser appear to be an array of lower power lasers for rf purposes provided that it still looks like one high power laser for optical purposes. Gas lasers are commonly excited by either a direct current (dc) or a radio-frequency (rf) discharge, typically 10-150 MHz.
With dc excitation the “longitudinal” discharge is typically between two metal electrodes, a cathode (−) and an anode (+) placed at opposite ends of a hollow glass tube containing the low-pressure gas mixture. These “laser tubes” bear a resemblance to household fluorescent lighting tubes. A high electric field, typically 5-20 KV/m is required to “strike” the discharge between the electrodes. Once the discharge is struck, a slightly lower (50-80%) voltage is required to sustain the discharge. The current flowing through the discharge from anode to cathode is limited and regulated by a series “ballast” resistor.
With rf excitation the “transverse” discharge is typically between two closely spaced (1-5 mm) metal plates with discharge lengths of up to 1 m or so. An rf voltage is applied across the plates via an impedance matching network to transform the discharge impedance to equal the combined rf power supply output impedance and coaxial cable delivery characteristic impedance for maximum power transfer efficiency.
The laser output power can be increased by length scaling of the dc- and rf-excited discharges at about 1 W/cm for the carbon dioxide (CO2) laser, for example. With rf-excited CO2 lasers the output power can also be increased by area scaling of the discharge at about 1 W/cm2. Area scaling of dc-excited CO2 lasers can be problematic because of the tendency of the dc discharge to filament into a “lightning bolt” arc. Area scaling of rf-excited lasers is less problematic by virtue of the tendency of the rf discharge to spread out to fill the electrode area. For example, a 100W CO2 laser can be excited by a 1 m dc- or rf-excited discharge length, or a 100 cm2 rf-excited discharge area.
As the rf-excited CO2 laser is area scaled to high powers of several hundred watts with several hundred cm2 of discharge area, the discharges can “arcs out.” One explanation for this is as follows: There are three types of rf discharge that can exit, the desired “alpha” discharge, an undesired “gamma” discharge and a catastrophic “arc”, as described in “The characteristics and stability of high power transverse radio frequency discharges for discharge CO2 slab laser excitation”, Vitruk et al, J. Phys. D: Appl. Phys. 25, 1776-1776, 1992. The alpha and gamma discharges are characteristic of the Townsend alpha and gamma coefficients for primary and secondary electron emissions from the electrode surfaces. The arc discharge is characteristic of thermionic emission from the electrode surfaces. The discharge impedances, in decreasing order of impedance are the alpha, gamma, and arc. For low power CO2 lasers up to 100W or so, the impedance discrimination between the alpha and gamma discharges is sufficiently great to ensure that the alpha discharge predominates. The impedance discrimination between a gamma discharge and an arc can be small and once initiated, the gamma discharge is capable of chemically and mechanically altering the electrode surface with discoloration and “pitting”. This can increase the likelihood of the gamma discharge recurrence and its transition to an arc, especially during discharge initiation (“striking”).
Unfortunately, as the discharge area is increased the alpha discharge impedance can decrease to a value closer to the gamma discharge value where there is less impedance discrimination between the two. The gamma discharge is a local area event and its area can be independent of the electrode area, so that several gamma discharges can exist simultaneously between two electrodes. For example, a gamma discharge might cover an area of 1 cm2 with impedance ten times less than a 1 cm2 alpha discharge, but equivalent impedance for a 10 cm2 alpha discharge. A local area of alpha discharge can collapse into a smaller area of gamma discharge with equivalent impedance. This hypothesis is supported by the observation of alpha and gamma discharges existing alternately between a pair of electrodes in a CO2 laser under similar operating conditions. The collapse may occur at a threshold rf power density (W/cm3) that is a function of the operating conditions. The conditions may include gas pressure, gas mixture, electrode separation, rf frequency, rf power supply output impedance characteristics, the cable length (mismatch standing wave) between power supply and discharge, and the interelectrode voltage determined by the impedance matching network. In a similar way, a local area of alpha discharge collapses into a smaller area of arc with equivalent impedance. In this way the current can be delivered to a full alpha discharge between the electrodes which can be “funneled” into an arc with the same impedance, but with the catastrophic loss of laser power and electrode surface destruction. FIG. 16 illustrates a typical electrode configuration with a discharge gap, and FIG. 17 illustrates a typical capacitance value for the system shown in FIG. 16.